markj wrote:That's interesting. The L is sort of a good comparison for this. It has the earlier port timing and the CV carbs. Anyone fimiliar with the miliage it gets or how it compares to the other models.
Indeed ! The 'L' carburettors were a different specification BS40 to later models (pilot jets and throttle valves) so while not a direct comparison to, for example an 'A' , it would at least be a good compare to a 'J' or a 'K'.
Ian
If at first you don't succeed, just get a bigger hammer !
I'd like to see those 'real world' mileage figures you speak of too, but dont expect miracles for the L model. Mr Burney in OZ has a pristine stock L, so if he can come forward with numbers those would be much appreciated.
If conventional flat slides are better than the older round slides,where does the BS compare?
The way a bike is ridden has more of an impact on MPG numbers than most other variables. For example one road test had MPG at 37.7 average and 30.0 ridden hard for an L. Cycle got 40mpg out of an M. John Robinson in the UK managed 41.7 from UK gallons average and 35.8 ridden hard, again on an M. Motorcycle Collector managed 41.9 recently on a restored L. A US test claimed approx 45 for a K. Cycle World claimed 44 for the J. Charles Dean at MCM managed 45 miles to a UK gallon average from a J.
So there doesn't seem to be a huge difference across models. It looks like
Ridden as befits an older bike 45
Average 40
Ridden hard 30
Gearing makes a difference as does engine efficiency and how well the carbs are set up.
Jet for performance and the air:fuel ratio will be around 12.6:1. Jet for economy and it will be closer to 15.4:1. So jetting makes a huge difference too, if the bike has been modified.
Add in some wear and tear and things not quite right and take off 10-20% from those numbers.
Fuel tank is 4.5 gallons or about 4 to reserve means 180 ridden nicely to 120 ridden hard - assuming there's only half a gallon left as it goes onto reserve.
Suzsmokeyallan wrote:..... and the VMs are worlds better than the quirky BS40 carbs........
While it is a fact the porting, exhaust characteristics and gearing changed, what is less clear is what the actual 'facts' are about the carburettors, as generally when this discussion comes up all you hear is opinion. I think you also would first have to decide what it was you planned to measure - performance or fuel economy as (and this is my own opinion not supported by fact ) I don't think you can get both at the same time with these engines.
I would dearly like to see some actual data showing a test engine with the carburettors being the only variable changed, in the same way it would be interesting to know how only changing from points to electronic ignition changes things. Has anyone ever actually done some science on this topic ?
So true Ian .. its a balance on what you want from your GT as getting both from a 2 stroke is just about impossible . You can have a fun GT thats still gets fair gas millage from a stock 48 something HP bike but if the desire is more sporting when you are pushing the 100+ HP range you must figure the MPG will drop a bunch when the engine isnt "on the pipe " you will be dumping unburned fuel out the pipe .. but the thrill of the 2 stroke on the pipe is something to behold
The 'L' carburettors were a different specification BS40 to later models (pilot jets and throttle valves) so while not a direct comparison to, for example an 'A' , it would at least be a good compare to a 'J' or a 'K'. 
